doc: use include for lib readme

doc/lib.rst

-Knot DNS Resolver library
-=========================
-
-Requirements
-------------
-
-* libknot_ 2.0 (Knot DNS high-performance DNS library.)
-
-Overview
---------
-
-Resolution plan
-~~~~~~~~~~~~~~~
-
-Cache
-~~~~~
-
-Reputation
-~~~~~~~~~~
-
-Modules
-~~~~~~~
+.. include:: ../lib/README.rst
 
 API reference
 -------------
 
 .. doxygenindex::
    :project: libkresolve
-
-.. _libknot: https://gitlab.labs.nic.cz/labs/knot/tree/master/src/libknot

lib/README.rst

-../doc/lib.rst
\ No newline at end of file

lib/README.rst

+Knot DNS Resolver library
+=========================
+
+Requirements
+------------
+
+* libknot_ 2.0 (Knot DNS high-performance DNS library.)
+
+Overview
+--------
+
+Resolution plan
+~~~~~~~~~~~~~~~
+
+Cache
+~~~~~
+
+Reputation
+~~~~~~~~~~
+
+Modules
+~~~~~~~
+
+.. _libknot: https://gitlab.labs.nic.cz/labs/knot/tree/master/src/libknot